1. Field of the Invention
The present invention generally relates to a sintered, hard material featuring a high degree of hardness and wear resistance, such as sliding-wear resistance, abrasive resistance and erosion resistance, and more particularly to a sintered, hard material of high hardness suitable for use in a variety of wear resistant tools, such as waterjet cutting nozzles, draw dies, dies and the like, which require high degrees of hardness and the aforesaid wear resistance.
2. Description of the Related Art
As a sintered hard material for use in the aforesaid wear resistant tools, there have generally been employed sintered hard alloys comprising a hard material phase such as of WC, TiC, TiN, TiCN, TaC and the like, and 4 to 20 wt % of a binder phase of an iron group metal such as Co and the like, the hard material phase sintered with the binder phase.
The aforesaid sintered hard alloy containing 4 to 20 wt % of iron group metal, such as Co, as the binder phase provides high degrees of strength and tenacity but a Micro-Vickers hardness of only about less than 2200 kgf/mm.sup.2 measured using a hundred gram load (hereinafter, simply referred to as "OMHv (0.1)"). Where such a sintered hard alloy is used as a material for forming wear resistant tools requiring high degrees of wear resistance like waterjet cutting nozzles, a sufficient wear resistance for practical use cannot be attained.
In this connection, there have been proposed sintered hard materials directed to improvement in hardness by reducing a mixing ratio of the iron group metal serving as the binder phase. For instance, Japanese Unexamined Patent Publication No. 5(1993)-230588 has disclosed a sintered hard material accomplishing improvement in hardness by reducing the content of Co serving as the binder phase to 2 wt % or less and employing WC powder having an average particle size of 2 .mu.m or less.
Unfortunately, the sintered hard material disclosed in this Patent Publication contains an amount of binder phase of a soft iron group metal and therefore, the hardness MHv(0.1) thereof is only increased to about 2700 kgf/m.sup.2. Furthermore, the above hard material falls short of providing a practically sufficient wear resistance for forming the waterjet cutting nozzles and the like, although it is not known if such a drawback results from local small portions of low hardness remaining in the material.
On the other hand, there has been suggested a sintered hard material free from a metal binder phase or containing no iron group metal serving as the binder phase for the purpose of achieving a further increase in the hardness.
The elimination of the metal binder phase from the hard material leads to a likelihood of appearance of W.sub.2 C and free carbon (C) additionally to a WC phase, as indicated by a W-C phase diagram of Sara, Rudy, J. T. Norton, et al. This is because there exists no iron group metal serving to form a solid solution with excessive W and C. Hence, it is extremely difficult to produce a sintered material of a single WC phase in a stable manner. As a consequence, a sintered material of a WC+W.sub.2 C phase or a WC+C phase is obtained.
In the case of a sintered material of the WC+C phase, the sintered material is much reduced in the overall hardness due to an extremely low hardness of free carbon and hence, is far from serving for the purpose of forming the waterjet cutting nozzles or the like.
As to the sintered material free from the metal binder phase, study was also conducted on the sintered material of the WC+W.sub.2 C phase, as suggested by Japanese Unexamined Patent Publication Nos. 4(1992)-365558 and 5(1993)-209248.
Where no metal binder phase is employed, as described above, it is difficult to obtain a dense sintered material due to the absence of a liquid phase in the sintering process. The absence of the liquid phase also leads to direct contact between WC and W.sub.2 C particles which, in turn, form a solid phase bond therebetween and hence, local formation of coarse grains results. As a consequence, the overall hardness of a resultant sintered material is reduced.
The density of a sintered material may be increased to a degree by subjecting the ingredients to increased sintering temperatures and pressures. Under such conditions, however, the grain growth of tungsten carbide becomes excessive. Hence, despite the merit of the absence of a soft metal binder phase, the resultant sintered material as a whole has a lower hardness than expected. In addition, heavy wear may sometimes arise from pores remaining in the sintered material. As a result, the resultant sintered material cannot achieve a wear resistance stable enough for practical use.
In this connection, Japanese Unexamined Patent Publication No. 4(1992)-365558 teaches the addition of a carbide or nitride of Ti, Ta, V, Cr and the like for suppressing the grain growth of tungsten carbide while allowing for the reduction of the sintering temperature.
According to this method, the grain growth of WC in the tungsten carbide component is retarded to a degree but the grain growth of W.sub.2 C in the tungsten carbide component cannot be reduced to about as low as that of WC. Consequently, the resultant sintered material cannot attain an adequate improvement in the overall hardness, thus failing to achieve such a high hardness value MHv(0.1) of over 3000 kgf/mm.sup.2.